The present invention is in the field of medical implants made of polyethylene (PE), in particular, ultrahigh molecular weight PE (UHMWPE) and high molecular weight PE (HMWPE).
Ultrahigh molecular weight polyethylene (hereinafter referred to as xe2x80x9cUHMWPExe2x80x9d) is commonly used to make prosthetic joints such as artificial hip joints. Wear of acetabular cups of UHMWPE in artificial joints introduces many microscopic wear particles into the surrounding tissues. The reaction to these particles includes inflammation and deterioration of the tissues, particularly the bone to which the prosthesis is anchored. Eventually, the prosthesis becomes painfully loose and must be replaced. It is generally accepted by orthopaedic surgeons and biomaterials scientists that the reaction of tissue to wear debris is the chief cause of long-term failure of such prostheses.
The literature describes numerous attempts to improve the wear resistance of polyethylene (hereinafter referred to as xe2x80x9cPExe2x80x9d) in joint replacements. Grobbelaar et al. [J. Bone and Joint Surgery, 60-B(3): 370-374 (1978)] attempted to improve the cold-flow characteristics of xe2x80x9chigh-densityxe2x80x9d PE prostheses made of Hostalen RCH 1000 C, without sacrificing its low-frictional properties, through a process of radiation crosslinking. Grobbelaar et al., crosslinked the PE using gamma radiation in the presence of crosslinking gases, including acetylene and chlorotrifluoroethylene, or in an inert nitrogen atmosphere. Due to the absorption of the crosslinking gasses, the surface was more crosslinked than the interior of the PE. Nevertheless, because of the high penetration power of gamma radiation, the PE became crosslinked throughout.
To improve the wear resistance of a medical prosthetic device, Farrar, WO 95/21212, used plasma treatment to crosslink its wear surface. This wear surface comprises a plastic material such as UHMWPE. Crosslinking was assumed to have occurred based on the presence of Fourier transform infrared absorption bands at 2890 cmxe2x88x921. Farrar claims his ATR (attenuated total reflection) data imply that he had achieved a penetration depth of 0.5 microns, but the degree of crosslinking is not disclosed.
Streicher, Beta-Gamma 1/89: 34-43, used high penetration gamma radiation or high penetration (i.e., 10 MeV) electron beam radiation to crosslink UHMWPE and high molecular weight PE (hereinafter referred to as xe2x80x9cHMWPExe2x80x9d) specimens throughout their entire thickness. Streicher annealed the gamma irradiated material in a nitrogen atmosphere in order to increase crosslinking and reduce oxidation during long-term storage. Streicher found that the wear of the materials was treater after the crosslinking by electron beam radiation.
Higgins et al (Transactions of the 42nd Ann. Mtg., Orthopaedic Res. Soc., Feb. 19-22, 1996, p. 485) attempted to stabilize UHMWPE against oxidation after high penetration gamma irradiation (which crosslinked their specimens through the entire thickness) by reducing the concentration of free radicals. They used the following post-irradiation treatments: (1) pressurizing in hydrogen at 15 psi for 2 hours, or (2) heating at 50xc2x0 C. for 182 hours. They compared the amount of free radicals remaining in the PE using electron spin resonance (ESR), but they did not assess the impact of these treatments on the mechanical or wear properties of the UHMWPE, nor on the oxidation resistance.
One aspect of the invention presents surface-gradient crosslinked (hereinafter also referred to as xe2x80x9csurface-crosslinkedxe2x80x9d) PE and medical implants (hereinafter abbreviated as xe2x80x9cimplantsxe2x80x9d) having surface-crosslinked PE which are wear resistant. The PE is preferably UHMWPE or HMWPE. Non-limiting examples of the invention are acetabular cups made of surface-crosslinked PE, and surface-crosslinked PE tibial components of knee prostheses. The surface-crosslinked PE and implants may be made by the methods described below.
Another aspect of the invention presents a method for improving the wear resistance of the bearing surface of an implant. The implant or its bearing surface is made of surface-crosslinked PE, the PE is preferably UHMWPE or HMWPE. In one embodiment, the method comprises exposing the implant to an electron beam (the term xe2x80x9celectron beamxe2x80x9d is hereinafter referred to as xe2x80x9ce-beamxe2x80x9d) radiation with an energy level specifically selected to crosslink the bearing surface of the implant, to improve the wear resistance, only to a depth sufficient such that the crosslinked layer will not be worn through during the life of the patient, while leaving the remainder uncrosslinked, thereby avoiding any reduction in mechanical properties that might otherwise result from crosslinking. Additionally, confining the crosslinking to a thin surface layer facilitates neutralizing residual free radicals caused by irradiation and/or extracting residual chemicals in the case of chemical crosslinking, as described.
In another embodiment of the invention, instead of crosslinking the bearing surface with e-beam radiation, the bearing surface of the implant is crosslinked to a limited depth with a free radical generating chemical, again while leaving the remainder of the implant uncrosslinked for the. reasons mentioned above. The free radical generating chemical is preferably a peroxide.
In both of the above methods, the crosslinking is preferably in the surface layer, gradually decreasing to nearly zero in the interior of the PE.
With e-beam crosslinking, it is preferable that the implant be packaged in a low oxygen atmosphere during irradiation, such as an inert gas (e.g., nitrogen) or a vacuum, in order to minimize oxidation and maximize crosslinking of the surface layer. However, if an implant is e-beam irradiated while in air, the outer layer of the bearing surface may then be removed, e.g., by machining, to eliminate the more oxidized and less crosslinked material. In such a case, the depth of crosslinking penetration of the e-beam can be increased by the appropriate amount to take into account the thickness of the material to be removed.
It is preferable that the surface-crosslinked material be treated to eliminate residual free radicals generated by the e-beam crosslinking process in order to stabilize it against long-term oxidation. This can be achieved by one or more of the following methods: (1) remelting the partially formed cross linked material after crosslinking irradiation but prior to final shaping into the implant. In this case, either the surface destined to be the bearing surface of the implant is remelted, or the whole partially formed crosslinked material is remelted, (2) annealing the partially formed crosslinked material or the final shaped implant, (3) exposing the crosslinked material or implant to pressurized hydrogen and/or (4) treating the crosslinked material or implant using ethylene oxide.
With chemical crosslinking, the implant may be annealed after crosslinking to stabilize its size and shape and/or the implant may be soaked in suitable solvent(s) to extract from the crosslinked surface layer any residual chemicals, resulting from decomposition of the free radical generating chemical, in order to minimize leaching out of such chemicals during in vivo use, and to minimize long-term oxidation of the crosslinked material. The soaking step may be conducted either before or after the annealing step.
In another aspect of the invention, instead of annealing, the chemically crosslinked implant is remelted to stabilize it against long-term oxidation. This can be achieved, e.g., by remelting the partially formed crosslinked material after crosslinking but prior to final shaping into the implant. In this case, either the surface destined to be the bearing surface of the implant is remelted, or the whole partially formed crosslinked material is remelted. If remelting is employed, the implant need not be annealed after crosslinking to stabilize its size and shape, since the remelting would also stabilize the size and shape of the implant. The implant or the partially formed crosslinked material may additionally be soaked in suitable solvent (s), as described above, to extract from the crosslinked surface layer any residual chemicals, either before or after remelting and/or reshaping of the implant.
Another aspect of the invention presents a method of treating a patient in need of an implant. An implant, or an implant with a bearing surface, composed of the surface-crosslinked PE is applied to the body of the patient, e.g., surgically implanted into the patient. The surface-crosslinked PE of the present invention has reduced wear rate. In the preferred embodiment, the bearing surface of the implant is composed of the surface-crosslinked PE. Known surgical methods for introducing an implant into a patient may be used.